The present invention relates to mechanical presses, and in particular to double action presses wherein a blanking and forming operation is performed on each cycle of the press.
In making can ends, for example, one slide of the press performs the blanking operation, which punches out a circular blank from strip stock, and the other slide forms the circular blank into a can end by means of a drawing operation. Thus, both operations can be performed on a single cycle of the press.
Due to the fact that the blanking operation must be carried out prior to the drawing operation, presses of this type are constructed such that the blanking slide will lead the forming slide. Furthermore, it is necessary to hold the blanked part during at least a portion of the drawing operation so as to prevent wrinkling. Some prior art presses have accomplished this by permitting the blanking punch to travel through the tin line and continue to exert pressure on the blanked part during the drawing operation. By permitting the blanking punch to overtravel past the cutting edge, excessive wear of the cutting edge is caused by the punch sliding past it. Unless proper clearance between the punch and cutting edge can be maintained at all times, metal to metal contact between the punch and cutting edge will occur during the overtravel portion of the cycle. A further disadvantage to overtravel of the blanking punch is that the punch will necessarily have a longer residence time beneath the tin line than will the forming die, and this delays ejection of the finished part.
In order to avoid overtravel of the blanking punch, certain types of presses drive the blanking punch slide by means of a cam. The cam is manufactured so that the blanking punch is caused to dwell at the bottom of its stroke at a point just below the tin line. This maintains pressure on the blanked part yet avoids overtravel so that the blanking punch can begin its return stroke at the same time as or just before the return stroke of the forming die. In a cam driven press of this type, the mechanism is quite complicated. Moreover, the fact that the contact between the cam and connecting rod is only a line contact, lubrication is difficult and the cam surfaces are subject to wear.